A highly sensitive and specific radioimmunoassay has been developed for the major structural protein of an oncornavirus etiologically associated with bovine lymphosarcoma. This test can be used to identify cattle which have been exposed to bovine leukemia virus and may thus develop or transmit the disease. Sera from 100% of the cattle with clinically-diagnosed lymphosarcoma exhibited high titered antibodies against the bovine leukemia virus major structural protein. Analysis of randomly obtained sera indicated that infection of this virus is widespread among cattle. BIBLIOGRAPHIC REFERENCES: Premkumar-Reddy, E., Devare, S. G., Vasudev, R. and Sarma, P. S.: A simplified radioimmunoassay for viral antigens: use of Staphylococcus aureus as an adsorbent for antigen-antibody complexes. J. Natl. Cancer Inst. (In press, 1977). Devare, S. G., Chander, S., Samagh, B. S. and Stephenson, J. R.: Evaluation of radioimmunoprecipitation of bovine leukemia virus infection in domestic cattle. J. Immunol. (In press, 1977).